Dear Liz: I am from Germany. I have had a bank account in America for over one year. Now I get my FICO score. After six months it was 738, half a year later, it was 771 and one month after that, 759. Why does it change in such a short time? Is it the real FICO score?

Answer: Welcome to the U.S. and its sometimes-baffling credit scoring systems. Even people who were born here often misunderstand how credit scores work.

You don’t have just one score; you have many, and they change all the time to reflect the changing information in your credit reports. Higher or lower balances on a credit card, a new credit application or the simple passage of time can make the numbers change.

The FICO scoring system is the most dominant, but lenders also use VantageScore, a FICO rival created by the three credit bureaus (Equifax, Experian and TransUnion), plus proprietary scores.

You also will see different numbers depending on which credit bureau report is used to create the score and which version of the score is used. Credit scoring formulas may be designed for certain industries and formulas are updated over time.

So your FICO Auto Score 6 from Experian likely won’t be the same as your FICO 4 from TransUnion, your FICO Bankcard Score 4 from Equifax or your VantageScore 3 from any of the bureaus, even if you get all the scores on the same day.

It can be hard to predict which score a lender will use, but the same behaviors tend to be rewarded by all of them. Those behaviors include paying bills on time, using only a small portion of your available credit, having different types of credit (installment loans and revolving accounts, such as credit cards) and applying for new credit sparingly.

If you’re using a score to monitor your credit, it’s important to use the same kind from the same bureau — otherwise you’re comparing apples and oranges, as we say in English.